Coating removal method and guide for use therewith

ABSTRACT

The present invention relates to a method for removing a coating from a defined area of a high pressure turbine part comprising the steps of: securing a guide to said turbine part, said guide both exposing said defined area while protecting adjacent areas; repeatedly inserting and withdrawing a brush through an opening in said guide which directs said brush to impinge said defined area and remove said coating. The invention also relates to a guide used in the aforementioned method.

BACKGROUND OF THE INVENTION

(i) Field of the Invention

The present invention relates generally methods for manufacturing suchparts which permit selective inclusion of coatings where desired on theone hand, but removal of such coatings at certain portions where theymight interfere with the mechanical fitting and functioning of the part,such as at dovetail joints of a high pressure turbine engine.

(ii) Description of Related Art

High pressure turbines (“HPT” s), such as those used in aircraftengines, as well as other dynamoelectric machines, such as generatorsused with gas and steam turbines, typically employ parts which aremechanically joined using dovetails. In aircraft engines, it is commonto employ dovetail joints on turbine rotor disks. In generators,dovetails are typically located at the top of the radial slots, and maybe used in conjunction with slides, wedges and ripple springs that aidin retaining windings.

To protect HPT's and other machine parts subject to extremes oftemperature and pressure, it is common to “paint” onto the partsanti-corrosion protective coatings. Typical anti-corrosion coatingsinclude ceramic coatings such as Alseal 700 or Alseal 589 Tan,manufactured by Coatings for Industry, Inc. of Souderton, Pa. Typically,the anti-corrosion coatings are applied to the parts prior to theirassemblage. However, in joining component parts together to form amechanical assembly, there are certain pressure face areas where thepresence of the anti-corrosion coating is detrimental in terms of thecloseness of the tolerances. For example, the presence of theanti-corrosive coating deleteriously affects dovetail joints in HPTs,where direct contact between the component parts provides a betterconnection.

In seeking to resolve this issue, it was initially contemplated tomodify the coating process so as to not apply coating to the pressureface areas of the article. However, it was found to be too difficult tocontrol the application process so as to both sufficiently coat thoseareas where the corrosion-resistant coating was desired while, at thesame time, sufficiently masking the pressure face areas to preventapplication of the coating thereon. Techniques for removing the coatingsuch as high pressure waterjet stripping and milling the coating fromthe pressure face areas were found ineffective in situations requiringprecision and selectivity in coating removal.

SUMMARY OF THE INVENTION

The present invention addresses the above described problem in that itapplies the corrosion-resistant coating to those portions of the partwhere it is needed for protecting the part from high temperature andpressure environments while, at the same time, providing little or nocorrosion-resistant coating on the pressure face areas of the part. As aresult of having no corrosion-resistant coating at pressure face areasof the parts, junction areas between the parts exhibit bettertolerances.

In a more specific embodiment, the part is an HPT rotor disk such asthose employed in aircraft engines and the pressure face area is adovetail joint. However, it will be appreciated that the method of theinvention is applicable to any HPT part, such as those employed ineither aviation or power generation.

In a first aspect, the present invention relates to a method forremoving a coating from a defined area of a high pressure turbine partcomprising the steps of:

-   -   securing a guide to the turbine part, wherein the guide both        exposes the defined area while protecting adjacent areas; and    -   repeatedly inserting and withdrawing a brush through an opening        in the guide which directs the brush to impinge the defined area        and remove the coating.

In a second aspect, the present invention relates to a method forremoving a corrosion-resistant coating from the dovetail slot of an HPTrotor comprising the steps of:

-   -   securing a guide to the dovetail slot by inserting and wedging        the guide inside the dovetail slot including the defined coating        removal area; and    -   repeatedly inserting and withdrawing a brush through an opening        in the guide which directs the brush to impinge the defined area        of the dovetail and remove the coating.

In a third aspect, the present invention relates to a guide comprising acontoured side surface adapted to interface with the teeth of a dovetailjoint of a rotor; a top surface having a lip adapted to hold the guidein place against one side of the rotor and at least one hole positionedover a guide channel, wherein the guide channel includes at least oneopening that exposes a surface of the dovetail joint when the guide isfitted to the rotor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a High Pressure Turbine Disk including a dovetail slothaving a guide inserted therein.

FIG. 2 depicts the guide, showing a series of holes for passage of thebrush.

FIG. 3 depicts a brush at the point of insertion into one of the holesof the guide inserted into the dovetail slot of the high pressureturbine disk.

FIG. 4 depicts the brush attached to the machinery which controls itsinsertion into and deletion from one of the holes in the guide insertedin the dovetail of the high pressure turbine disk.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The method of the present invention is generally applicable to theremoval of deposits from a defined area on a component, such as ananti-corrosion coating from a dovetail pressure face on turbine rotordisk in a jet engine. The process is particularly effective in removinganti corrosion coating deposited on an HPT disk, in which the coating isremoved from a specifically defined area such as the pressure faces ofthe dovetail slots. This method is effective at maintaining closetolerances for the perimeter of the defined area without disturbing theremaining coating adjacent to the defined area.

The anti-corrosion coatings suitable for application to HPT's aregenerally known in the art. They generally include inorganic ceramicbinders which permit the coatings to endure the extreme temperatures andenvironments that exist in stationary and aviation gas turbines.Commercially available coatings suitable for use in the presentinvention include ALSEAL 700 and ALSEAL 598 Tan, or similar paint-likematerials.

As depicted in FIG. 1, the invention relates to the treatment of the HPTdisk 1, which includes around its circumference a number of dovetailslots 2. To carry out the method of the invention, a guide 3 is insertedand wedged into the dovetail slot 2 including six pressure faces.

An embodiment of the guide 3 is depicted in greater detail in FIG. 2.FIG. 2 shows three of the six openings corresponding to the pressurefaces of a HPT turbine, labeled 5 a, 5 b, and 5 c. The openings allow abrush to impinge upon only that area of the pressure face where removalof the coating is desired. The guide 3 further includes six guide holes,6 a, 6 b, 6 c, 6 d, 6 e and 6 f. As shown in FIG. 3, the guide holes 6a-f are used to guide the brush 4 to engage the surface of one of thesix pressure faces of the HPT disk 1. When properly inserted and wedgedinto the dovetail slot 2, the guide 3 defines a coating removal area,exposing only the defined area while protecting the adjacent areas.

The guide also controls the degree of impingement and the defined area.The defined area, as discussed above, relates to that area from which itis desired to remove the coating, such as dovetail pressure faces. Theguide 3 provides a physical covering over those areas of the surfacewhere it is desired to retain the coating.

The guide 3 needs to be strong enough so as not to break down whenexposed to the physical stresses of the reciprocating action of thebrush strokes, as well as strong enough to maintain its integrity so asto protect those areas of a component, such as an HPT, to preventcoating removal. Generally both metals, as well as plastics, such aspolycarbonates, have been found to be suitable in this regard, thoughother plastics could be employed. The guide can be manufactured by useof 3-d printing methods known in the art or by other methods. It will beappreciated that the geometry of the guide can be tailored to anysurface of a component where it is desired to remove a selected portionof the coating thereof in accordance with the method of the presentinvention. The guide 3 is typically inserted and wedged securely intothe piece to be treated, such as the dovetail slot 2 containing thecoating removal area.

The brush 4 is depicted at FIG. 3 at the point of insertion into theguide 3. The geometry of the brush 4 generally relates to the size ofopening in the guide 2 that the brush 4 reciprocates through on enteringand withdrawing from the guide orifices 6 a-6 f The diameter has to besmall enough to permit entry and withdrawal of the brush 4 into and outfrom the guide orifices 6 a-6 f while at the same time being largeenough to exert a certain amount of force on the coatings so as to beable to abrade all or substantially all of the coating from the selectedarea. Generally, the diameter of the brush preferably ranges from 2 to 5mm and more preferably about 2.5 mm. The brushes themselves generallyinclude a central shaft 7 having bristles 8 about its circumference. Inthe embodiment shown in FIG. 3, the bristles 8 form a spiral patternabout the central shaft 7.

The bristles 8 can be made of any materials with suitable strength towithstand the reciprocating insertion and withdrawing from the orificesas well as suitably abrading qualities to remove coating from the deviceupon impingement with that coating. The material also needs to havesuitable flexibility so that the bristles can bend in the orifice asthey impinge on the selected area for the coating removal. Personsskilled in the art will understand how to choose metals with sufficientstrength and flexibility to provide suitable abrading of a surface inthe context of the present invention. A particularly preferred metal forthe process is stainless steel for both the bristles and the centralshaft.

As depicted in FIGS. 3 and 4, the central shaft 7 can be attached to achuck 9 which firmly houses and holds the central shaft and creates thereciprocating up and down motion to insert and withdraw the brush 4 intoand out of the orifices 6 a-6 f of the guide 2, thereby achieving thedesired abrading at the exposed areas of the device to achieve removalof all or substantially all of the coating therefrom. The controlledmethod utilizes the guide 2 to force the brush to impinge the coatingremoval area to a controlled amount such 0.015 inch. The method strokesthe brush in and out through the guide 2 at a controlled rate of speedranging between 20 and 60 inches per minute and preferably at about 40inches per minute. In a preferred embodiment, the method indexes acylindrical brush a controlled amount after every stroke cycle toprovide even brush wear, e.g., by rotating the brush after each cycle.An exemplary brush index is about 36 degrees after each stroke cycle,though it will be appreciated that this can be higher or lower.

In a preferred embodiment, the method tracks the stroke cycles toprovide the proper number of strokes per coating removal area tocompletely remove the coating from the area. The method also preferablytracks the brush usage to a predetermined brush life cycle. For example,the method advantageously employs as a control a CNC machine such as astandard CNC milling machine. The method also works manually, though amore automated method is preferred.

By virtue of the method of the present invention, the integrity of thesurrounding adjacent coating, as evidenced by highly magnified metal, ismaintained [Any illustration of this we could include in theapplication?]. On the other hand, the method effectively removes all orsubstantially all of the coating from selected areas.

EXAMPLE

A guide was designed by use of a CNC machine to fit into the dovetailjoints on a turbine rotor disk as depicted in FIG. 1. The guide is madeof polycarbonate and includes six orifices adapted to receive a brush.Each orifice corresponds to a dovetail section which includes one ormore coatings to be removed by the brush. In the embodiment illustrated,each orifice was 1-2 mm in diameter, preferably 1.5 mm, and the lengthof the dovetail section on the turbine rotor, corresponding to thelength of a corresponding section on the guide, was 30-40 mm in length,preferably 35 mm. The brush was made of stainless steel, and included abristle portion being at least as large as the length of the guide. Theshaft itself had diameter less than that of the orifice and the bristleshad diameter slightly greater than that of the orifice and at leastlarge enough to make contact with the surface of the dovetail sectionwhen inserted into the orifice. The brush portion was formed as a spiralabout the axis of the shaft. In operation, the machine reciprocated thebrush into and out of each orifice at a rate of 40 inches/minute, whilethe shaft was rotated 36° after each insertion to ensure even wear ofthe brush. Inspection of the dovetails on the turbine rotor disk aftercompletion of the process revealed that the corrosion-resistant paintcoating had indeed been removed from the defined area, whereas the guidehad effectively protected the coating in the areas protected by theguide. The result obtained was better as compared to where the definedareas were protected before spraying the corrosion-resistant paintcoating onto the part, which was unexpected.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal language of the claims.

What is claimed is:
 1. A method for removing a coating from a definedarea of a high pressure turbine part comprising the steps of: securing aguide to said turbine part, said guide both exposing said defined areawhile protecting adjacent areas; repeatedly inserting and withdrawing abrush through an opening in said guide which directs said brush toimpinge said defined area and remove said coating.
 2. The method ofclaim 1, wherein said coating a corrosion-resistant coating.
 3. Themethod of claim 2, comprising painting said corrosion-resistant coatingonto said high pressure turbine to form the coating.
 4. The method ofclaim 1, wherein said defined area is a dovetail slot.
 5. The method ofclaim 4, comprising securing said guide in said dovetail slot byinserting and wedging said guide inside the dovetail slot containing thedefined coating removal area.
 6. The method of claim 1, wherein saidguide comprises plastic or metal.
 7. The method of claim 1, wherein saidbrush comprises stainless steel bristles.
 8. The method of claim 7,wherein said bristles have a diameter between 1 and 3 mm.
 9. The methodof claim 8, wherein said bristles have a diameter of 2.5 mm.
 10. Themethod of claim 7, wherein a defined length of said brush bristlesimpinge on said defined area.
 11. The method of claim 10, wherein saiddefined length is 0.3 to 0.5 mm.
 12. The method of claim 12, whereinsaid defined length is 0.38 mm.
 13. The method of claim 1, comprisingstroking the brush in and out through the guide at a rate of speedbetween 30 and 50 inches per minute.
 14. The method of claim 14, whereinsaid brush is cylindrical and indexed by a controlled amount after everystroke.
 15. The method of claim 15, wherein the brush is indexed by 36degrees after each stroke.
 16. The method of claim 2, wherein thecorrosion-resistant coating is a ceramic coating.
 17. The method ofclaim 1, wherein said high pressure turbine part is a rotor disk.
 18. Aguide for removing a coating from a surface of a dovetail joint of arotor comprising: a contoured side surface adapted to interface with theteeth of a dovetail joint of a rotor; a top surface having a lip adaptedto hold the guide in place against one side of the rotor and at leastone hole positioned over a guide channel, wherein the guide channelincludes at least one opening that exposes a surface of the dovetailjoint when the guide is fitted to the rotor.
 19. The guide of claim 18,wherein the side surface of the guide includes six cavities adapted tointerface with the six teeth in the dovetail joint of the rotor and sixguide channels corresponding to each tooth of the rotor.
 20. The guideof claim 18, wherein the guide includes a cutout portion of the topsurface.